# 1146. Snapshot Array

https://leetcode.com/problems/snapshot-array

## Description

Implement a SnapshotArray that supports the following interface:

* `SnapshotArray(int length)` initializes an array-like data structure with the given length.  **Initially, each element equals 0**.
* `void set(index, val)` sets the element at the given `index` to be equal to `val`.
* `int snap()` takes a snapshot of the array and returns the `snap_id`: the total number of times we called `snap()` minus `1`.
* `int get(index, snap_id)` returns the value at the given `index`, at the time we took the snapshot with the given `snap_id`

**Example 1:**

```

**Input:** ["SnapshotArray","set","snap","set","get"]
[[3],[0,5],[],[0,6],[0,0]]
**Output:** [null,null,0,null,5]
**Explanation:** 
SnapshotArray snapshotArr = new SnapshotArray(3); // set the length to be 3
snapshotArr.set(0,5);  // Set array[0] = 5
snapshotArr.snap();  // Take a snapshot, return snap\_id = 0
snapshotArr.set(0,6);
snapshotArr.get(0,0);  // Get the value of array[0] with snap\_id = 0, return 5
```

**Constraints:**

* `1 <= length <= 50000`
* At most `50000` calls will be made to `set`, `snap`, and `get`.
* `0 <= index < length`
* `0 <= snap_id <`(the total number of times we call `snap()`)
* `0 <= val <= 10^9`

## ac

```java
```
